An O2 smart plastic film for packaging

Abstract

The preparation and characterisation of a novel, water-proof, irreversible, reusable, UV-activated, O₂ sensitive, smart plastic film is described. A pigment, consisting of a redox dye, methylene blue (MB), and a sacrificial electron donor, DL-threitol, coated onto an inorganic support with semiconductor functionality, TiO₂, has been extruded in low-density polyethylene (LDPE). The blue-coloured indicator is readily photobleached in <90 s using UVA light (4 mWcm⁻²), whereby MB is converted to its colourless, leuco form, leuco-methylene blue (LMB). This form persists in the absence of oxygen, but is re-oxidised to MB in ∼2.5 days in air under ambient conditions (∼21°C, ∼65% RH) within the O₂ smart plastic film. The rate of recovery is linearly dependent upon the ambient level of O₂. At the lower temperature of 5°C, the kinetics of the photobleaching activation step is largely unchanged, whereas that of recovery is markedly reduced to t_1/2 = 36 h at 5°C (cf. 9 h at 21°C); the activation energy for the recovery step was calculated as 28 kJmol⁻¹. The O₂-sensitive recovery step was found to be moderately dependent upon humidity at 21°C, but not significantly dependent upon humidity at 5°C. The possible application of this type of indicator in food packaging is illustrated and discussed briefly.